1. Technical Field
The present disclosure relates to pixel structures for an autostereoscopic display apparatus. More particularly, the present disclosure relates to pixel structures for an autostereoscopic display apparatus used in televisions, computer monitors, telecommunications handsets, digital cameras, laptop and desktop computers, games apparatuses, automotive and other mobile display applications.
2. Description of Related Art
Normal human vision is stereoscopic, that is each eye sees a slightly different image of the world. The brain fuses the two images (referred to as the stereo pair) to give the sensation of depth. Three dimensional (3D) stereoscopic displays show a separate image to each of the eyes corresponding to that which would be seen if viewing a real world scene. The brain again fuses the stereo pair to give the appearance of depth in the image.
FIG. 1 shows in plan view a display surface in a display plane 201. A right eye 202 views a right eye homologous image point 203 on the display plane and a left eye 204 views a left eye homologous point 205 on the display plane to produce an apparent image point 206 perceived by the user behind the screen plane. If light from point 203 is seen by the eye 204 and light from the point 205 is seen by the eye 202 then a pseudoscopic image point 210 is produced. Pseudoscopic images are undesirable as they produce visual strain to observers.
FIG. 2 shows in plan view a display surface in a display plane 201. A right eye 202 views a right eye homologous image point 207 on the display plane and a left eye 204 views a left eye homologous point 208 on the display plane to produce an apparent image point 209 in front of the screen plane. Pseudoscopic image point 212 is produced if the eye 202 can see light from point 208 and the eye 204 can see light from point 207.
FIG. 3 shows the appearance of the left eye image 10 and right eye image 11. The homologous point 205 in the left eye image 10 is positioned on a reference line 12. The corresponding homologous point 203 in the right eye image 11 is at a different relative position 203 with respect to the reference line 12. The separation 13 of the point 203 from the reference line 12 is called the disparity and in this case is a positive disparity for points which will lie behind the screen plane. Similarly in the left eye image 10, the homologous point 208 is positioned on a reference line 14 while in the right eye image the corresponding homologous point 207 is laterally separated from the reference line 14 by a distance 15 with a negative disparity. Changing from the left eye image 10 to the right eye image 11 as shown in FIGS. 1 to 3, the movement of the homologous point 203 is to the right. This corresponds to an apparent image point 206 behind the screen plane, while the movement of the homologous point 207 is to the left, corresponding to an apparent image point 209 in front of the screen plane.
For a generalized point in the scene there is a corresponding point in each image of the stereo pair as shown in FIG. 3. These points are termed the homologous points. The relative separation of the homologous points between the two images is termed the disparity; points with zero disparity correspond to points at the depth plane of the display. FIG. 1 shows that points with uncrossed disparity appear behind the display and FIG. 2 shows that points with crossed disparity appear in front of the display. The magnitude of the separation of the homologous points, the distance to the observer, and the observer's interocular separation gives the amount of depth perceived on the display.
Stereoscopic type displays are well known in the prior art and refer to displays in which some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be color filters in which the images are color coded (e.g. red and green); polarizing glasses in which the images are encoded in orthogonal polarization states; or shutter glasses in which the views are encoded as a temporal sequence of images in synchronization with the opening of the shutters of the glasses.
Autostereoscopic displays operate without viewing aids worn by the observer. In autostereoscopic displays, each of the views can be seen from a limited region in space as illustrated in FIG. 4.
FIG. 4 shows a display device 16 with an attached parallax element 17. The display device 16 produces a right eye image 18 for the right eye channel. The parallax element 17 directs light in a direction shown by the arrow 19 to produce a right eye viewing window 20 in the region in front of the display. An observer places their right eye 22 at the position of the window 20. The position of the left eye viewing window 24 is shown for reference. The viewing window 20 may also be referred to as a vertically extended optical pupil.
FIG. 5 shows the left eye optical system. The display device 16 produces a left eye image 26 for the left eye channel. The parallax element 17 directs light in a direction shown by the arrow 28 to produce a left eye viewing window 30 in the region in front of the display. An observer places their left eye 32 at the position of the window 30. The position of the right eye viewing window 20 is shown for reference.
The parallax element 17 acts as an optical steering mechanism. The light from the left image 26 is sent to a limited region in front of the display, referred to as the viewing window 30. If an eye 32 is placed at the position of the viewing window 30 then the observer sees the appropriate image 26 across the whole of the display 16. Similarly the optical system sends the light intended for the right image 18 to a separate window 20. If the observer places their right eye 22 in that window then the right eye image will be seen across the whole of the display. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution.
In this application the term “3D” is used to refer to a stereoscopic or autostereoscopic image in which different images are presented to each eye resulting in the sensation of depth being created in the brain. This should be understood to be distinct from “3D graphics” in which a 3D object is rendered on a two dimensional (2D) display device and each eye sees the exact same image.
The parallax element 17 may be switchable between a state in which it provides a 3D image and a state in which it has substantially no optical effect to allow selective display of 3D and 2D images. In this application the term “2D/3D” is used to refer to a display apparatus in which the function of the optical element can be so switched to enable a full resolution 2D image or a reduced resolution autostereoscopic 3D image.
FIG. 6 shows in plan view a display apparatus comprising a display device 16 and parallax element 17 in a display plane 34 producing the left eye viewing windows 36, 37, 38 and right eye viewing windows 39, 40, 41 in the window plane 42. The separation of the window plane from the display is termed the nominal viewing distance 43. The windows 37, 40 in the central position with respect to the display are in the zeroth lobe 44. Windows 36, 39 to the right of the zeroth lobe 44 are in the +1 lobe 46, while windows 38, 41 to the left of the zeroth lobe are in the −1 lobe 48.
The viewing window plane 42 of the display represents the distance from the display at which the lateral viewing freedom is greatest. For points away from the window plane, there is a diamond shaped autostereoscopic viewing zone, as illustrated in plan view in FIG. 6. As can be seen, the light from each of the points across the display is beamed in a cone of finite width to the viewing windows. The width of the cone may be defined as the angular width.
The parallax element 17 serves to generate a directional distribution of the illumination at the window plane 42 at a defined distance 43 from the display device 16. The variation in intensity across the window plane 42 constitutes one tangible form of a directional distribution of the light.
If an eye is placed in each of a pair viewing zones such as 37, 40 then an autostereoscopic image will be seen across the whole area of the display. To a first order, the longitudinal viewing freedom of the display is determined by the length of these viewing zones.
The variation in intensity 50 across the window plane of a display (constituting one tangible form of a directional distribution of the light) is shown with respect to position 51 for idealised windows in FIG. 7. The right eye window position intensity (or luminance) distribution 52 corresponds to the window 41 in FIG. 6, and intensity distribution 53 corresponds to the window 37, intensity distribution 54 corresponds to the window 40 and intensity distribution 55 corresponds to the window 36. The integrated intensity distribution 60 is the sum of the intensity of the individual windows 52, 53, 54, 55 and further adjacent windows.
FIG. 8 shows the intensity distribution with position schematically for more realistic windows. The right eye window position intensity distribution 56 corresponds to the window 41 in FIG. 6, and intensity distribution 57 corresponds to the window 37, intensity distribution 58 corresponds to the window 40 and intensity distribution 59 corresponds to the window 36. The ratio of the variation from a nominal intensity distribution of the distribution 60 to the nominal intensity in an angular range is termed the angular intensity uniformity (AIU). The nominal intensity distribution may be for example a flat Luminance distribution as shown in FIG. 7, a Lambertian distribution, or some other distribution with a substantially smoothly varying intensity profile. The AIU may be measured over a limited range of viewing angles, or over the entire angular range of output angles of the respective display.
FIG. 9 shows a further intensity distribution in which substantially triangular shaped windows 61 are overlapped in order to produce a flat distribution 60. Advantageously, such windows can provide a robust means by which to reduce fluctuations in the distribution 60. Further such windows reduce image flipping artefacts in which the image content appears to rapidly change from one view to another, causing an apparent rotation of the image to an observer.
Several 3D artefacts can occur due to inadequate window performance, particularly for overlapping windows. Pseudoscopic images occur when light from the right eye image is seen by the left eye and vice versa. This is a significant 3D image degradation mechanism that can lead to visual strain for the user. Overlapping windows are seen as image blur, which limits the useful amount of depth that can be shown by the display. Additionally, poor window quality will lead to a reduction in the viewing freedom of the observer. The optical system is designed to optimize the performance of the viewing windows.
In displays with multiple views, adjacent windows contain a series of view data. As an observer moves laterally with respect to the display, the images seen by each eye vary so that the appearance of a 3D image is maintained. Human observers are sensitive to variation in luminance as they move with respect to the display. For example, if the distribution 60 varies by more than 0.5%-5% of the total, then the display will appear to flicker. Thus it is desirable to minimize the variation of the distribution 60. As the distribution varies with the viewing angle, the uniformity of the distribution may be referred to as the angular intensity uniformity (AIU) which is an important performance parameter.
The respective images are displayed at the display plane 34, and observed by an observer at or near the window plane 42. The variation in intensity across the window plane 42 is not defined by the variation in intensity across the image. However, the image seen by an observer at the window plane 42 may be referred to as the image at the viewing window for ease of explanation.
There will now be discussed some known techniques for improving the AIU of a display.
One type of prior art pixel configuration for autostereoscopic display apparatus uses the well known stripe configuration as shown in FIG. 11a as used for standard 2D displays. The pixels apertures 62 are arranged in columns of red pixels 65, green pixels 67 and blue pixels 69. To generate an autostereoscopic display, a parallax element 63 such as a lenticular array is aligned with groups of color pixels 65, 67 and 69 as shown. The cusp 71 between the lenses of the array is one example of the geometric axis of the array of parallax elements.
The parallax element 63 may be slanted so that the geometric axes of the optical elements (e.g. lenses in the case of a lenticular array) of the parallax element 63 are inclined to the vertical column direction of the pixel apertures 62, as described for example in U.S. Pat. No. 3,409,351 and U.S. Pat. No. 6,064,424. Such an arrangement enables overlapping of windows, similar to that shown in FIG. 9, that results in a better uniformity of the distribution 60 of intensity compared to a parallax element 63 in which the geometric axes of the optical elements are parallel to the vertical column direction of the pixel apertures 62.
Herein a line parallel to the geometric axes of the optical elements of a parallax element is termed a “ray line” (being a line along which rays of light are nominally (ignoring aberrations) directed from a display device to the same relative horizontal position in the window plane at any one vertical position in the window plane, rather than being the direction of a ray of light). FIG. 11a further shows the inclined orientation of the ray lines 64 and the geometric axes of the optical elements of the parallax element 63 with respect to the pixel apertures 62. Such an arrangement will generate windows that are tilted with respect to the vertical such that the view data will appear to change as the observer moves vertically.
FIG. 11a further includes a graph of the resultant intensity 50 at horizontal positions 51 in the viewing (window) plane for an ideal lens with an idealized focused spot. For ease of understanding, the positions where a ray line 64 crosses the distribution 60 correspond to horizontal position 51 into which light is directed from the ray line 64. The intensity 50 has an intensity which is generally flat but which has peaks 74 whose origin has been appreciated as follows.
The intensity distribution at each given position 51 can be determined by measuring the total intersection length 66, 68, 70, 72 of the ray lines 64 corresponding to that position 51 with the aperture across adjacent pixel apertures 62. This is because, in operation, the parallax element 63 collects light from a ray line 64 and directs it all towards a position in space where that light is observed by the viewer (in fact an eye receives light from a bundle of ray lines 64 due to the pupil size, lens aberrations and lens focus condition so the actual intensity observed is a convolution of the intensity distribution 60 but this will still have similar peaks). Thus as the intensity 50 varies, the total intersection length varies due to the ray line 64 varyingly covering different amounts of the pixel apertures 62 and the gaps therebetween. In particular, the distribution 60 includes elevated levels where the total intersection length is high because the ray line 64 intersects more of the pixel apertures 62 in the corner thereof.
As can be seen, the total intersection length 66, 68, 70, 72 can include contributions from two adjacent pixel apertures 62. While these adjacent pixels have two different colors, each will have a corresponding pixel of the same color in the unit cell structure of the 3D image. So, the adjacent pixels can conveniently be used to form an understanding of the total intersection length within a single color.
In some known systems with non-uniform distribution 60 where the parallax element 63 is a lenticular array, the lenses may be defocussed in order to smooth the distribution 60, effectively by providing an average of the different intersection lengths 66 of different ray lines 64. However, such an approach creates an increased overlap between the 3D windows and results in increased levels of image blur, reduced useful depth and increased pseudoscopic images. It is desirable to maintain a high AIU without increasing the defocus of the lenses.
WO-2007/031921 discloses a technique by which the increases in the distribution 60 are reduced by means of a pixel cut-out 76 as shown in FIG. 11b. (As an aside, similar considerations apply in the teachings of WO 2005/006777 which describes pixel arrangements in which the pixel shape is modified so as to provide improved uniformity in displays in which the optical elements are aligned parallel to the pixel columns.) The cut-out 76 compensates for the increased intersection which otherwise occurs in the corner of the pixel aperture 62 reducing the total intersection length for those ray lines 64 and thereby flattening the intensity distribution 60. However, such an arrangement cannot be used to compensate the output of wide viewing angle displays, as will be shown below.
Conventional Liquid Crystal Display (LCD) panels such as twisted nematic Liquid Crystal (LC) with homogeneous alignment use substantially rectangular pixel aperture shapes in which the whole of the pixel operates as a single domain such that the angular contrast properties of the optical output are substantially constant for each part of the pixel. Such pixels are well suited to the rectangular cutout approach to improve uniformity of distribution 60. However, such panels suffer from significant variations of contrast with viewing angle freedom due to the restrictions of the optical performance of a single liquid crystal alignment domain within the cell. In order to compensate for such viewing angle effects, one approach is to use Vertical Aligned (VA) LC materials in combination with multiple domain structures and further complex alignment modification techniques. In this case each pixel comprises plural domains having different alignments of the liquid crystal molecules in each domain. The contrast viewing angle properties of the display are determined by the addition of contrast properties from the individual domains.
The appearance of the output illumination function from such pixels is shown in FIGS. 12a to 12b. FIG. 12a shows that the individually addressable red pixels 82, green pixels 84 and blue pixels 86 typically comprise top and bottom sub-pixels 88, 90 with transmitting apertures 92 and 94 respectively. Within the sub-pixels 88, 90, the transmitting aperture is divided into a first group of domains 1, 2, 3, 4 for the top sub-pixel and a second group of domains 5, 6, 7, 8 for the bottom sub-pixel. The sub-pixels 88, 90 are separated by regions 98, 100 containing electrodes, capacitors and other addressing circuitry. The upper and lower sub-pixels 88, 90 may be addressable unitarily or may be addressable separately.
FIG. 12b illustrates in more detail the domain structure of the sub-pixels 88, 90 by showing the relative location of each of the domains 1-8 within the top sub-pixels 88 and bottom sub-pixels 90. Each of the domains 1-8 contributes to optimum viewing contrast properties in a certain quadrant of the output of the display. Some of the domains such as 3, 4 occupy more than one portion of sub-pixel 88 and 7, 8 occupy more than one portion of sub-pixel 90. Domains 1, 2, 5, 6 are termed as ‘main domains’ while domains 3, 4, 7, 8 are termed as ‘sub-domains’. The contribution of each of the domains 1-8 to the total output is defined by the active area of the domain. The relative contributions of each domain may be adjusted to improve the angular contrast uniformity, for example to improve image washout performance at high angles. For example, the main domains 1, 2, 5 and 6 may have the same area and the sub domains 3, 4, 7 and 8 may have the same area greater than the area of the main domains 1, 2, 5 and 6. The ratio of the area of a main domain to the area of a sub domain is termed as the domain area ratio and for example may be between 3:7 and 1:1, and typically 9:11.
Applying the cut-out method to the individually addressable display elements in which the aperture is the shape remaining from a rectangular footprint when one or more cut-outs is removed will not produce the required uniformity with viewing angle, for example, as is illustrated in FIGS. 13a and 13b. FIG. 13a shows the alignment of ray line 64 with VA pixel structure of FIG. 12. As described previously, such an arrangement would provide non-uniform output intensity because of variation in the total intersection length of different ray-lines with position across the pixel width. FIG. 13b shows the introduction of cut-outs 102, 103 into the individually addressable pixel apertures.
It has been appreciated that, although such an arrangement might be expected to provide some improvement to uniformity, it will not produce high uniformity in a VA LC display device wherein the domains are required to operate uniformly in order to maintain the contrast properties across different viewing angles of the display apparatus. As the uniformity of each of the domains 1-8 is not controlled, there will be variation in the contrast with angle. This property of the uniformity of the contrast with viewing angle may be termed the angular contrast uniformity (ACU). The non-uniformity is illustrated in FIG. 14 showing how the distribution 60 for an individual domain 1 suffers from AIU degradation, this resulting in ACU degradation for the display apparatus as a whole. In particular the intersection length 104 of a ray line 64 with the domain 1 produces a flat region 106 in the AIU distribution 60 across much of the domain, but the region 108 of the domain 1 aperture will produce a defect 110 in the AIU distribution 60. This means that while the integrated intensity of the pixel as a whole may be uniform by aperture considerations alone, the domain 1 uniformity will not be constant with the consequence that the display apparatus will appear to vary in intensity, particularly in the quadrant for which domain 1 has a dominant contrast effect on the final image quality.
Thus, the use of cut-outs in the pixel aperture does not deal with maintenance of ACU in display apparatuses having multiple domains, for example wide viewing freedom displays such as VA displays, because the AIU of individual domains is not considered.